1. Field of the Invention
The present invention relates to an electric circuit, and more particularly, to a charge pump for generating a high voltage required for an internal circuit of a semiconductor memory device.
2. Description of the Related Art
As the need for larger dynamic random access memory (DRAM) integrated circuits (ICs) increases, the integration of the DRAM IC increases, and the amount of control circuitry required on the DRAM increases. Accordingly, the DRAM ICs consume more power. When such semiconductor memory devices are in portable systems, the devices consume a large amount of power and quickly drain the batteries of the portable systems. Accordingly, the increasing integration of semiconductor memories shortens the time for which the portable system can operate. In response, the supply voltage of semiconductor memory devices has been decreased to decrease power consumption. For example, in the past, semiconductor memory devices had a supply voltage of 5.0 volts. However, more recent semiconductor memory devices use a supply voltage of 3.3 volts, and development of devices having supply voltages lower than 3.3 volts is continuing.
A Metal oxide semiconductor (MOS) transistors are common in semiconductor memory devices. When the supply voltage passes through an n-channel MOS (or NMOS) transistor, the voltage drops. More specifically, when the supply voltage is applied to the drain and the gate of an NMOS transistor, the resulting source voltage is less than the supply voltage by the threshold voltage of the NMOS transistor. Using a gate voltage that is higher than the supply voltage can keep the gate-to-source voltage equal to the threshold voltage of the NMOS transistor and provide a source voltage equal to the supply voltage.
In general, charge pumps include a plurality of MOS transistors for generating and transmitting a high voltage. It is possible to obtain a higher voltage from the charge pumps by reducing the loss of voltage due to the threshold voltage of the MOS transistors in transmitting the high voltage. However, efficient charge pump circuits are sought and are essential for semiconductor memory devices that operate at voltages at or below 2.5 volts.